A fiber optic transmission channel is typically made up of a transmitter, a receiver, and an optical waveguide linking these two components. In short-haul applications, a common waveguide is a laser-optimized multimode optical fiber that is rated to some standard like, for example, the OM3 or OM4 standards specified in ISO/IEC 11801 and ANSI/TIA-568-C.3. OM3 rated fibers have a minimum effective modal bandwidth of 2000 MHz·km and OM4 rated fibers have a minimum effective modal bandwidth of 4700 MHz·km. These fibers are often paired with VCSEL-based transmitters and receivers (commonly referred to herein as “transceivers”) with emission wavelengths usually around 850 nm±10 nm.
To help ensure the performance of these channels, standard organizations like IEEE 802.3 and INCITS T11 specify the worst-case operational parameters for the transceivers and fiber channel links. However, using worst-case multi-mode fibers (MMFs) to evaluate transceivers, especially in a high volume manufacturing environment, presents significant challenges. Generally speaking, for a given alpha-refractive index profile, a wide range of effective modal bandwidth (EMB) and differential mode delay (DMD) values are produced within a drawn length of fiber due to manufacturing process variations during the fiber preform vapor deposition. Typically, bare MMF is spooled in lengths of 8.8 km or 17.6 km for the cabling operation, and bare fiber test samples are cut from both ends of the spooled bare fiber. In this case, test samples range in length from 300 m to 1000 m. An alternative method is to measure the EMB and/or DMD over the total length of the spooled fiber, where the EMB represents the average value. Based on these measurements, the fiber spools are classified as OM3, OM4 or Wideband MMF. However, both test methods have deficiencies. Due to bandwidth variation along the spooled bare fiber, testing short end pieces or the average over the total length does not guarantee the bandwidth at any given segment along the fiber. Depending on the manufacturing process variation, the actual value of a short fiber segment (<1000 m) along the spooled fiber can vary significantly. Experiments, using a state of the art DMD/EMB measurement system indicates the EMB and DMD can vary from 5% to 30%. As such, due to the manufacturing variability and low yield for fibers having a bandwidth within the critical region for the worst-case scenario, obtaining a fiber with the particular characteristics can be problematic.
While such problem may be dealt with by an extensive and costly evaluation of numerous fibers, a more critical problem is caused by an incomplete description of the fiber dispersion phenomena in the link modes utilized in the estimation of the worst-case channels. The link models utilized in industry standards assume that the modal and chromatic dispersion do not interact and therefore the sign of the MMF differential mode delay (DMD) does not have an effect on the performance of the channel. This assumption, however, is incorrect, and the interaction between the modal and chromatic dispersion can significantly affect the overall performance of the fiber. As a result, a fiber that may be considered to be a worst-case scenario fiber may, after all, perform considerably better than expected. This can lead to inaccurate test results for transceivers, and thus there is a need for improved methods, systems, and apparatuses designed to enable more accurate optical transceiver testing.